Textures split up when using "Data Transfer" modifier
Delete Data Transfer modifier.
Create a UV map.
Create a new image in the image editor.

In the low poly object's material nodes, click the blue thing to make it a separate material from the original object's.
Unlink the image from the Principled BSDF and select the new image.

In the original material, plug the texture into the Material Output.

Select the original object then the low poly object so that it is active.
Make sure the disconnected image node is selected in the node editor.

In Properties Editor → Render Properties, set up the Bake section something like this.(This section is only available for CPU rendering.)

The Extrusion setting is the one to fiddle with.
After setting the render samples to a low number (1 worked for me), click Bake.

This should leave you with a nice new image texture that will work with the UV map that you created.